The present invention relates to an improved apparatus for the titration of gases, which can be used inter alia for the determination of the storage properties of metal hydrides.
The invention also relates to an apparatus hereinafter called xe2x80x9ccycling apparatusxe2x80x9d, which permits to evaluate the behaviour of a substance when this substance is subjected to a large number of gas absorption/desorption cycles. This cycling apparatus can be used inter alia for evaluating the degradation of storage properties of a metal hydride subjected to cycles of hydrogen absorption/desorption.
There are presently apparatuses especially devised for the titration of gases. These apparatuses are used in particular for determining the hydrogen absorption capacity and, therefore, the storage properties of metal hydrides. In the last case, they are particularly used for:
evaluating the storage capacity of metal hydrides as a function of the operating pressure (pressure=f(H/M) where H is the number of hydrogen atoms and M is the number of metal atoms); and
evaluating the absorption and desorption kinetics (reaction dynamics) of the metal hydrides [H/M=f(time)].
FIG. 1 schematically illustrates the structure of an example of an existing apparatus used for the titration of hydrogen. This apparatus is disclosed in an article of Pascal TESSIER entitled xe2x80x9cHydrogen storage in metastable Fexe2x80x94Tixe2x80x9d of 1995.
As can be noticed, this existing apparatus comprises a main duct 1xe2x80x2 which is connected by a valve V3xe2x80x2 to a source of hydrogen under pressure 5xe2x80x2, and on which is mounted a pressure sensor (manometer) 7xe2x80x2 for measuring the total pressure of hydrogen within the circuit.
The apparatus also comprises a first derivation duct 9xe2x80x2 which connects the main duct via a valve V6xe2x80x2 to a measuring chamber (13xe2x80x2) having the shape of a tube in which can be introduced a sample of the substance for which he absorption or desorption properties are to be measured. The tube 13xe2x80x2 is located in a furnace 11xe2x80x2 having a temperature that can be adjusted at will as a function of the measurement to be carried out.
The apparatus further comprises a second derivation duct 15xe2x80x2 having a first end 17xe2x80x2 connected to the main duct 1xe2x80x2 upstream of the connection between the same and the first derivation duct 9xe2x80x2, and a second end 19xe2x80x2 connected to the main duct downstream of the junction of the same with the first derivation duct. This second derivation duct 15xe2x80x2 includes a small tank 21xe2x80x2 of 50 cc and a differential pressure sensor 23xe2x80x2. A valve V11xe2x80x2 is located in the main duct 1xe2x80x2 between the junction 17xe2x80x2 and the first derivation duct 9xe2x80x2. Two other valves V5xe2x80x2 and V12xe2x80x2 are respectively located on the second derivation duct 15xe2x80x2 between, on the one hand, the tank 21xe2x80x2 and the junction 17xe2x80x2 and, on the other hand, the differential sensor 23xe2x80x2 and the junction 19xe2x80x2.
Last of all, the apparatus comprises a third derivation duct 27xe2x80x2 connecting a pump 29xe2x80x2 via a valve VIxe2x80x2 to the main duct 1xe2x80x2 upstream of the junction 17xe2x80x2.
The valves mentioned hereinabove are operated by an informatized control system 33xe2x80x2. The two sensor pressures 7xe2x80x2 and 23xe2x80x2 are also connected to the control system. Most of the components of the apparatus are insulated in an isothermal enclosure 35xe2x80x2 shown in dotted lines. A manual valve V10xe2x80x2 is located in the derivation duct 9xe2x80x2. This manual valve V10xe2x80x2 is kept permanently open except when the sample is inserted.
In use, after suitable calibration, one starts by creating a vacuum within the whole system by closing the valve V3xe2x80x2 and by opening all the other valves to connect all the ducts, the sample carrying tube 13xe2x80x2 and the tank 21xe2x80x2 to the pump 29xe2x80x2. Then, all the valves are closed and the measurement up is bean by adjusting the hydrogen source to a given pressure. The valve V3xe2x80x2 is opened and then closed. Subsequently, the valve V5xe2x80x2, V11xe2x80x2 and V12xe2x80x2 are opened in series. After a pause, the valve V5xe2x80x2 is closed and, after another pause, the valve V6xe2x80x2 is opened and the measurement is carried out by measuring all the data given by both pressure sensors 7xe2x80x2 and 23xe2x80x2.
This can be repeated several times with an increase in the hydrogen pressure, in order to obtain pressure/composition isotherm curves.
If the existing apparatuses for the titration of gases like the one disclosed hereinabove are efficient, they are subject to very stringent limitations in their use, because of their response time and the saturation of their differential pressure sensors, which reduces the limits of operation of the apparatus, its sensibility and the limits of detection of the same.
This problem is particularly important in that some metal hydrides like the nanocrystalline alloys disclosed in the following recently laid-open patent application Nos. CA-A-2,117,158 and WO-A-96/23906 naming the Applicant as one of the coowners, have very fast absorption and desorption kinetics.
From a practical standpoint, it is possible to increase the operating range of the apparatus by modifying the sequences of opening of the admission valves. However, the equilibrium time of the system is slower, which leads to a substantial lost of data at the beginning of each measurement.
Accordingly, there is presently a real need for an apparatus for the titration of gases where the response time would be improved and the differential pressure sensor would be less subject to saturation, with the major drawback that such limits generate, namely a diminution of the range of use of the apparatus, expressed in amount of metal hydride needed for a given sensitivity threshold and maximum working pressure, both in PCT mode [pressure=f(H/M)] and in dynamic mode [(H/M=f(time)].
On the other hand, there are presently no apparatus available on the market, at least to the knowledge of the Applicant, which would permit to carry out rapidly and in an efficient manner, titration measurements at two different pressures and two different temperatures, in order to characterize a substance like an hydride, and more precisely, the efficiency of this hydride when it is subjected to a large number of hydrogen absorption /desorption cycles.
It has already been proposed to use conventional titration apparatuses for this purpose. However, because of the delays that are relatively long for achieving furnace temperature equilibrium as well as the pressure equilibrium (a reequilibrium is required at reach cycle), these apparatuses are poorly adapted for cycling, where it is necessary to change the temperature of the furnace as well as the pressure quickly between each cycle during the course of measurements.
Therefore, there is also the need for a cycling apparatus which would permit to carry out absorption/desorption cycles at two temperatures and two operating pressures in a fast, efficient and performing manner.
The present invention satisfies the two needs mentioned hereinabove by providing:
on the one hand, a new apparatus for the titration of gases having an improved response time, a more important dynamics range relative to the amount of powder that is used and to the maximum operating pressure and an improved sensitivity; and
on the other hand, a cycling apparatus allowing a substantial reduction of the time required for the analysis and determination of the properties of absorbing or desorbing materials during a large number of absorption/desorption cycles.
The apparatus according to the invention for the titration of a gas comprises:
a main duct (1) connected by a valve (V3) to a source of gas under pressure (5), said main duct being also connected to a first pressure sensor (7a);
a first derivation duct (9) connecting the main duct (1) via a valve (V6) to a sample carrying tube (13) which is located in a furnace (11) of adjustable temperature and is devised to receive a sample of a substance having gas absorption or adsorption/desorption properties to be measured;
a second derivation duct (15) having ends (17,19) connected to the main duct, at least one (19) of said ends being downstream of the first derivation duct (9), said second derivation duct connecting in series a valve (V5), a tank (21) and a differential pressure sensor (23);
a third derivation duct (27) connecting a pump (29) via a valve (V1) to the main duct (1);
an isothermal enclosure (35) for keeping the ducts and valves at a stable and controlled temperature; and
a control system (33) for adjusting and controlling at will the temperature of the furnace (11), the pressure of the gas and the valves in real time.
This apparatus is characterized in that it further comprises:
a fourth derivation duct (37) connected via a valve (V7) to a reference tube (39) which has the same characteristics as the sample carrying tube and is located together with the same in the furnace (11), said fourth derivation duct being connected to the second duct (15) between the tank (21) thereof and the differential pressure sensor (23).
As can be appreciated, the titration apparatus according to the invention differs from the existing apparatuses in that it includes a reference tube within the furnace close to the sample carrying tube. The sample carrying tube and the reference tube are connected on both sides of the differential pressure sensor, thereby leading to a substantial increase in the general performances of the titration system.
Due to this structural difference, the titration apparatus according to the invention has three major advantages.
First of all, its range of use is wider with respect to the amount of powder and the maximum pressure that can be used.
Secondly, the sensitivity of measurements is increased (the limit of detection is improved).
Thirdly, its response time is faster (larger dynamics range and reduction in the equilibrium time required for the differential pressure sensor).
On the other hand, the apparatus according to the invention for the cycling of a gas absorbing/desorbing material, is characterized in that it comprises:
a furnace (111) with two compartments (171a, 171b) each having an adjustable temperature, said furnace being movable between two positions by suitable means (175);
a main duct (101) connected by a valve (V103) to the source of gas to be absorbed or adsorbed, this main duct being also connected to a pressure sensor (107);
a first derivation duct (109) connecting the main conduct (101) via a valve (V106) to a sample carrying tube (113) which is located within the furnace (111) in such a manner as to be always located in one of the compartments whatever be the position of the furnace, said sample carrying tube being in one of the compartments when the furnace is in one of its two positions, and in the other compartment when the furnace is in the other of its two positions;
two second derivation ducts (115a and 115b) independent from each other and connectable alternatively to the main duct (101) via two corresponding valves (V163a and 163b), each of said second derivation ducts (115) including a valve (V105), a tank (121) and a differential pressure sensor (123);
a third derivation duct (127) connecting a pump (129) via a valve (V101) to the main duct; and
two fourth derivation ducts (137a and 137b) each connecting one of the second derivation ducts via a valve (V107a, V107b) to a reference tube (139), said reference tubes (139a and 139b) of these fourth derivation ducts being positioned within the furnace in such a manner as to be each positioned in one of the compartments of the furnace whatever be the position of the same, one of the reference tubes being always associated to the sample carrying tube whatever be the compartment in which the latter is located.
As it can again be understood, the cycling apparatus according to the invention comprises two furnaces and two enclosures that are kept under different hydrogen pressures, and are connected to a simple yet efficient computerized interface. It permits to quickly carry out measurements under two different pressures and at two different temperatures, and therefore to evaluate the degradation of the storage capacity of a substance like a metal hydride that is subjected to absorption/desorption cycles.
As previously indicated, one of the main applications of these two apparatuses is for evaluating in a more efficient and precise manner, the properties of recent hydrogen storage materials. This efficiency is due to the fact that these apparatuses are particularly well adapted for the measurement of very fast absorption/desorption kinetics.
However, it is worth mentioning that these apparatuses can also be used for numerous other applications, such as the absorption/desorption of other gases, the absorption, for example, of natural gas, the evaluation of the problems of oxidation and reduction of materials, etc.
The invention and its advantages will be better understood upon reading the following non-restrictive description of two preferred embodiments of the invention given with reference to test results.